1. Field of the Invention
The present invention relates to a multi-dimensional periodic structure applicable to a polarizing optical element or a photonic crystal in the optical field, and particularly relates to a multilayer structure using a multilayer film.
2. Related Art
A photonic crystal has a texture whose refractive index changes with a period near the wavelength of light. It is expected that the unique properties of the photonic crystal can be used for achieving optical applications which could not be achieved heretofore. A polarizing property is one of the unique properties. It is expected that the polarizing property of the photonic crystal can be applied to a polarizing optical element which is an essential optical component in a wide technical field related to optical communication, display, optical recording, optical measurement, etc.
For the photonic crystal, it is necessary to produce a periodic structure with a very fine texture. As a method for manufacturing such a structure, there is known a method of laminating thin films of materials with different refractive indexes (e.g. see Japanese Patent Publications No. JP 2000-56133A and JP 2000-131522A). The method can comparatively easily form a two-dimensional or three-dimensional periodic texture by forming a very fine irregular texture in advance on the surface of a substrate, and forming thin films thereon so that the thin films reflect the shape of the substrate surface.
A method for forming concave portions in the substrate by etching is used as the method for forming a periodic irregular texture in the substrate surface. In the method, a mask having a periodic pattern is formed on the surface of a glass or silicon substrate by photolithography, and concave portions are formed in the glass or silicon substrate by vapor phase etching such as ion beam etching through the mask (see JP 2000-56133A).
This method has a disadvantage as follows. That is, the periodic texture is formed only by forming the thin films. In contrast, photolithography and etching are required for processing the substrate. Thus, the processing cost increases and the processing time is also long, resulting in a bottleneck in the whole production process.
As a solution to the problem, there is known a method in which a resin material is used for a substrate and a periodic irregular texture is formed on the substrate by molding (see JP 2000-131522A). A resin having flowability is molded by a molding tool and hardened. Thus, an irregular texture can be formed. The method has an advantage as follows. That is, as long as the molding tool can be processed accurately, the molding tool can be used repeatedly. Thus, irregular textures having a fixed shape can be mass-produced.
Resin is generally low in heat resistance. When such a resin is used as a material of a substrate, restriction on temperature for forming thin films on the substrate may prevent the thin films from being formed with good quality. Moreover, even after the formation of an element, there is a problem in weather resistance. That is, the element using such a resin is degraded easily by heat, humidity, light, etc.